Depends - Facebook did it to manipulate their feelings, if you change the color on a bottom to see which one gets the most clicks I don't think that would be considered unethical.
Still, there's no oversight for either type of experiment (Facebook's or the color change). Where the border gets drawn between ethical and unethical is murky even WITH oversight, let alone without it.
If that is what bothers you then we could form a group of a/b test reviewers for which companies could submit their tests to - it would still be voluntary but it would be easier to say after the fact that it had been a approved by an ethics board.
